Disk drive devices using various kinds of disks, such as an optical disk, a magneto-optical disk, and a flexible magnetic disk, have been known in the art. In particular, a hard disk drive (HDD) has been widely used as a storage device of a computer and has been one of indispensable disk drive devices for current computer systems. Moreover, the HDD has found widespread application such as a removable memory used in a moving image recording/reproducing apparatus, a car navigation system, a cellular phone, or a digital camera, as well as the computer, due to its outstanding characteristics.
Magnetic disks used in the HDD have a plurality of data tracks formed concentrically. Each data track has a plurality of servo data having address information and a plurality of data sectors containing user data recorded thereon. Between each servo data, a plurality of data sectors are recorded. A head element portion of a head slider supported by a swinging actuator accesses a desired data sector according to address information of servo data, which allows data write to and data retrieval from a data sector.
In order to increase recording density of a magnetic disk, it is important to decrease variations in clearance between the head element portion flying over the magnetic disk and the magnetic disk; some mechanisms have been proposed to adjust the clearance. One of such mechanisms has a heater in a head slider which heats the head element portion to adjust the clearance (for example, refer to Japanese Patent Publication No. 2006-269005 “Patent Document 1”). In the present specification, it is called thermal fly-height control (TFC). The TFC generates heat by applying electric current to the heater to make the head element portion protrude by thermal expansion. This reduces the clearance between the magnetic disk and the head element portion. Another mechanism has been known that uses a piezo element to adjust the clearance between the magnetic disk and the head element portion.
In an HDD having such a mechanism that adjusts the clearance between the head element portion and the magnetic disk or the clearance between the slider and the magnetic disk, it is required that a control value to achieve a proper clearance is specified. The control value is a heater control value such as heater power in the TFC or a voltage to be applied to a piezo element in the mechanism using the piezo element. The variations in the clearance between the head element portion and the magnetic disk or in the clearance corresponding to the control value are varied depending on the head slider. It is desirable that the control value in a read/write operation is individually set to each head slider (see Patent Document 1, for example).
A technique to specify a proper control value for each head slider varies the clearance between the head element portion and the magnetic disk to detect contact between the head slider and the magnetic disk. The proper control value for the head slider can be specified from the control value at the contact. Contact of the head slider to the magnetic disk can be detected from, for example, a sensed value of an acoustic emission sensor, a positional error signal, read signal strength of a read element, a control value of a voice coil motor, or the like.
Use of a specific detector like an acoustic emission sensor causes issues like increase in the number of components of the HDD and cost. On the other hand, positional error signals and read signal strength can be measured by normal functions of the HDD so that contact between the head slider and the magnetic disk can be detected without implementing a new circuit or the like in the HDD.
When positional error signals or signal strength is measured with servo signals, however, slight contact may not be detectable. Servo data are recorded discretely in a circumferential direction on the magnetic disk. If the oscillation cycle of the head slider due to contact with the magnetic disk is smaller than the cycle of discretely recorded servo data produced by the rotation of the magnetic disk, namely, if the oscillation frequency due to contact is greater than the reading frequency of the servo data, the oscillation due to contact cannot be properly detected from the servo signals. Typically, it is difficult to detect oscillation with the band width of a half or more of the band width of the servo data. Therefore, contact cannot be detected until greater contact or oscillation with a greater amplitude caused thereby (oscillation with a smaller frequency) arises.
It is therefore required to detect contact between the head slider (head element portion) and the magnetic disk within small contact and a short period so as to prevent the head element portion or the magnetic disk from being damaged due to the contact between the head slider and the magnetic disk in addition to accurately determine the control value at which the contact occurs. The amplitudes of read signals in user data between servo data can be measured at more points than in the servo data. Therefore, smaller contact can be detected in a shorter period.
However, the user data are recorded in data sectors being discrete from each other in a data track. The HDD cannot measure accurate amplitudes of read signals for detection of contact in a gap between data sectors and a control signal area included in a data sector. Since there are several data sectors between servo data in a typical HDD, it is required for the HDD to measure amplitudes of read signals avoiding the gap and the control signal area, or to measure amplitudes of read signals in an area including these areas and select values measured in the user data area from the measured values. This significantly restricts the effect in detection of contact using the amplitudes of read signals.